This is a nonprovisional application of Provisional Application No. 60/183,959 filed on Feb. 22, 2000, which is hereby incorporated by reference.
1. Field of Invention
The present invention relates to a noncorrective eyeglass lens as well as a pair of eyeglasses fitted with such a lens.
2. Description of Related Art
A noncorrective eyeglass lens generally has an external surface and an internal surface both of which are portions of a sphere. These two spheres have different radii and different centers. Gullstrand""s law enables the power of such a lens to be calculated as a function of the radii of the spheres, from the positions of the centers of these spheres, and from the index of the material used to make the lens. For a noncorrective lens, a power of zero is chosen so that there is a relationship between the radii of curvature of the surfaces, the positions of the centers of the spheres corresponding to these surfaces, and the index of the material used. Current technological means provide full control of the power of a lens and enable products to be made with power tolerances of approximately {fraction (1/100)} diopter.
A continuing concern in the production of noncorrective lenses is to minimize prism defects. These defects increase in size with the curvature of the lens. The present invention relates to such lenses. For a lens whose radius of curvature of the external surface is equal to a maximum of approximately 90 mm, prism defects are more problematic and the invention thus relates particularly but not exclusively to these lenses.
U.S. Pat. No. 1,741,536 describes the necessity of decentering the optical center relative to the visual center of a fitted lens to avoid prism defects. The basic idea is that the optical axis of the lens must always be parallel to the main visual axis. This US patent describes in particular the application of this principle to lenses with a large face angle requiring decentering on the horizontal plane.
U.S. Pat. Nos. 5,648,832 and 5,689,323 take up the principle stated in the previous patent, applying it to eyeglasses with large pantoscopic angles requiring decentering in the vertical plane of the optical center relative to the visual center. The pantoscopic angle can be defined as the angle between a vertical plane and a plane on which the edge of the lens rests when the eyeglass is in position on a wearer. These two patents teach decentering toward the top of the lens.
The use of vertical decentering toward the top of the optical center of a lens optimizes and even eliminates the prism defect at the visual center. The purpose of vertical decentering or positioning of the optical center toward the top of the lens is to meet various international standards that consider only the main visual axis as the measuring axis to evaluate the quality of eyewear equipped with noncorrective lenses. However, this optimization creates a substantial prism defect at the bottom of the lens. Thus, if the wearer of such eyeglasses looks at his feet for example by casting his eyes down rather than bowing his head, the prism defect of the lower part of the lens gives him the impression that the ground is slipping away underneath his feet. The more pronounced the curvature of the lens, the more marked is this phenomenon.
The present invention thus sets out to correct this defect and provide an eyeglass lens that decreases the prism deviation observed at the bottom of the lens without this prism deviation becoming large at another point on the lens.
For this purpose, it proposes a noncorrective eyeglass lens designed to be fitted into a frame having two earpieces and composed of two spherical surfaces and an optical center.
According to the invention, in the standard wearing position in which a horizontal line connects the center of the pupil of the eye to an area of the ear on which an earpiece of the frame rests, the optical axis defined by the line passing through the two centers of the spherical surfaces makes an angle of at least 10xc2x0 with a horizontal axis.
Such an eyeglass lens thus has a low prism value along the main visual axis. This low value remains within the tolerances accepted by profession-wide standards and is not annoying to the eyeglass wearer. On the other hand, the prism deviation applied by the lens to a light beam passing through the lower peripheral area is decreased: an eyeglass wearer looking down at his feet for example does not feel that the ground is slipping away from under him.
This result is true in particular for a lens with a strong curvature corresponding to a radius of curvature of no more than approximately 90 mm.
To complete this result, the lens according to the invention advantageously has a negative power of between xe2x88x920.12 and xe2x88x920.04 diopter. This power is preferably between xe2x88x920.06 and xe2x88x920.04 diopter. This low power is not perceptible to the eyeglass wearer but makes a substantial contribution to decreasing the prism defect in the bottom of the lens.
The lens thickness at the optical center is preferably between 1 and 3 mm so that thin lenses are obtainable.
The optical axis of the lens, in the fitted and worn position, is preferably inclined downward relative to the horizontal axis corresponding to a line passing from the pupil of the eyeglass wearer""s eye to the area of the ear on which the earpiece of the frame rests. This runs counter to the teachings of the above-cited patents in which the optical axis is substantially parallel to the horizontal axis defined above.
The invention relates in particular to an eyeglass having a frame with two earpieces and two lenses according to the invention.
In any event, the invention will be properly understood from the description hereinbelow referring to the attached schematic drawings that show one embodiment of an eyeglass lens according to the invention as a nonlimiting example.